The profound and intelligible documentation, describing API of the DjVu.js library and the viewer, must be here, but as you may see - there is nothing :)
The library is by no means well known and I have doubts that it's worth writing the documentation, since nobody except for me uses the library. I decided that I will write the documentation later or when somebody asks me to do it. So if you want to use the library and need the documentation, create an issue on GitHub or just write me at firstname.lastname@example.org (if I don't response - create an issue).
The technical documentation (how to build the library locally and so on) is available on GitHub
The short guide how to try out the viewer locally is on the Downloads page